Spring-mass systems may be used for many applications, particularly to form an energy recovery device, an accelerometer, an inclinometer, a seismometer, etc.
FIG. 1 is a partial simplified lateral view of an example of an energy recovery device 10 using a microelectromechanical system or MEMS. It for example is a vibration energy recovery device 10 with an electret-based electrostatic converter. Such vibration energy recovery devices particularly aim at converting the ambient vibrations into electricity, particularly to power low power consumption systems, for example, self-contained sensors or pacemakers. Device 10 comprises a plate 12, for example, made of a dielectric material, particularly glass, and a spring-mass system 14 mounted on plate 12. Spring-mass system 14 comprises a mass 18 mobile with respect to plate 12 and connected by springs 20 to an element 16 which is fixed with respect to plate 12. Springs 20 allow a displacement of mobile mass 18 relative to plate 12 along a direction (Ox), which is generally horizontal. Call direction (Oz) the direction perpendicular to direction (Ox) and to the surface of plate 12 supporting electrodes 26. Direction (Oz) generally corresponds to the vertical direction.
Spring-mass system 14 may be partly formed by etching of a plate of a semiconductor material, particularly silicon. In this case, mobile mass 18 may comprise a portion 22 of the silicon plate and an additional mass 24 attached to portion 22. Each spring 20 may comprise series-connected rectilinear beams arranged in zigzag.
Electrodes 26 are provided on lower plate 12 opposite mobile portion 22. Electrets 28 may be arranged on the surface of mobile portion 22 opposite electrodes 26. Electrodes 26, mobile semiconductor material portion 22, and electrets 28 form an electret-based electrostatic converter. When vibrations are applied to device 10, spring-mass system 14 converts these vibrations into an alternating motion of mobile mass 18 along direction (Ox). The relative motion between mobile portion 22 and plate 12 causes the flowing of an electric current in a load resistor placed across the electrostatic converter.
For the proper operation of energy recovery device 10, electrodes 26 and electrets 28 should not come into contact with one another and, preferably, the distance between electrodes 26 and electrets 28 should remain substantially constant. This means that the displacement of the mobile mass along direction (Oz) should be as small as possible, in particular much smaller than the possible displacement along direction (Ox), for example according to a ratio 1/10 or 1/100. Further, for the power provided for each displacement by energy recovery device 10 to be as high as possible, the displacement of mobile mass 18 along direction (Ox) should be as high as possible.
To enable to efficiently recover the ambient vibrations, the resonance frequency of the spring-mass system should be substantially equal to the frequency of these vibrations. Ambient vibrations have a frequency which is generally lower than 50 Hz. Accordingly, it is desirable for the spring-mass system to have a low resonance frequency, smaller than 50 Hz, or even than 20 Hz. Such resonance frequencies are easy to obtain with conventional mechanical systems of large dimensions, particularly having a surface area greater than 3 cm2 in a plane perpendicular to direction (Oz). Such resonance frequencies are much more difficult to obtain with MEMS structures made of silicon and having surface areas generally smaller than 3 cm2. Indeed, such MEMS structures require using a mobile mass 18 having a weight which may be greater than one gram and beams forming springs 20 having a width smaller than 100 μm.
This may cause a sinking of mobile mass 18 along direction (Oz) under the effect of its own weight, which may be incompatible with the proper operation of device 10, particularly by the placing in contact of electrodes 26 with electrets 28.